To thin or not to thin ...

Understanding dense eucalypt stands and the pros and cons of thinning

Stands of dense woody regrowth are increasing in extent across Australia and around the world, and that raises many questions on how they should be managed. What's their value and should we leave them alone or actively thin them?

Dense woody regrowth commonly pops up on cleared land where there has been some change in land use, usually a reduction in grazing pressure. In some places, this regrowth is considered a bad outcome.

In parts of Europe, for example, the grasslands that may have been grazed for centuries are considered valuable for biodiversity in their own right. In Australia, on the other hand, woody regrowth is often considered a good outcome for biodiversity as it represents a transition back to the pre-cleared vegetation state. However, it is common for these regrowth stands to be much denser than undisturbed forest. They are often structurally simplistic with a high density of similar sized stems. These stems grow more slowly than in natural systems due to competition for resources. And this competition also suppresses the understorey vegetation, which was the focus of our research.

In Victoria, there is an increasing call for management of dense eucalypt stands on both private and public land. The most commonly cited management option is thinning – cutting down a proportion of stems and applying herbicide to prevent regrowth. The theory is that the release from competition should make the remaining stems grow faster, larger, and broader, as well allowing the recovery of understorey vegetation.

Self-thinning does occur in these systems and, given enough time, dense stands are generally expected to improve in quality. However, this is far slower than with active intervention. At the most basic level, the questions for managers then are: How bad is a dense stand for biodiversity and what is the benefit of thinning? But perhaps more importantly, we need to ask whether thickets pose a problem that warrants major investment from government. At what scale would thickets need to be a dominant form to cause concern for those species, and communities, and at what scale is the treatment cost effective?

Chris Jones and colleagues sought answers to these questions using data from two separate field projects conducted in box-ironbark woodlands and forests in central Victoria, where they evaluated the vegetation structure of dense regrowth stands of eucalypts, and the effect of thinning management. In order to determine what density of stems and cover of understorey 'should' be expected in natural systems, they evaluated their results in relation to published benchmarks of stem density and understorey vegetation cover.

They found that stands with stem density greater than benchmark levels suppress native understorey vegetation cover below its benchmark levels. Thinning stems can restore native understorey vegetation (richness and cover) in the short term, providing the soil seedbank has not been removed and there is no excessive grazing. This is the desired outcome from thinning, but the catch is that BOTH native and exotic species can recover following thinning.